Negative feed-back circuit



Nov. 4, 1941. D. FOSTER NEGATIVE FEED-BACK CIRCUIT Filed Dec. 22, 1939 INVENTOR DUOLE' Y E BY w .psrs/z ATTO R N EY F'atented Nov. 4, 1941 I PATENT OFFICE NEGATIVE FEED-BACK CIRCUIT Dudley E. Foster, South Orange, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 22, 1939, Serial N11. 310,492

6 Claims; c1; 179-471) The present invention relates to an improved negative feed-back circuit. In known circuits of this type it has been found that distortion is not completely eliminated. The main object of this invention is to provide a negative or inverse feed-back circuit which satisfactorily eliminates distortion to a high degree, provides increased feed-back and results in increased speaker damp- The novel features characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its construction and mode of operation together with other objects and advantages thereof, will best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 discloses a negative feed-back circuit in accordance with my invention, and

Fig. 2 is an equivalent bridge circuit which will serve to explain the invention.

The circuit of Fig. 1 comprises a first tube T1 which is resistance-capacity coupled to a second tube T2, R2 being the plate resistance of T1 and Rg being the grid resistance of T2. R1'is a feedback resistance connecting the plate of T2 with the plate of T1 as wellas with the grid of T2 through the coupling condenser K. Re is anunbypassed cathode resistor for T2. The cathode and grid of tube T2 are connected together by means of the circuit which comprises the resistance R2, and condenser C. Preferably T2 is a power amplifier or output tube having its output coupled to a loud speaker L, R1. representing the load impedance of the tube T2.

In the usual type of parallel inverse feed-back circuit the cathode resistor Re is by-passed by the usual shunt capacity to eliminate inverse feedback from that source, and the path C--R2 is omitted. It we designate the equivalent resistance of R2 and R; in parallel as R4, the feed-back factor for the usual parallel feed-back circuit is Now this feed-back voltage is applied not only to the grid of T2, which is desired, but also to the plate of T1. This voltage feed-back from the plate of T2 to the plate of T1 (through R1) causes distortion in T1, although it reduces distortion in T2 by virtue of the feed-back action. This increase in distortion in T1 is because R1 acts as if it were reduced by the factor 1+G2, where G2 is the gain of T2. insofar as it affects the equivalent A. C. load impedance for T1. This low effective load impedance is particularly bad when T1 is a triode. When T1 is a pentode there is still an increase in distortion but not to as great a degree.

.5 By adding the circuit CR2, where C is a blocking condenser to eliminate any change in bias of T2 due to connection of R2, and by eliminating the by-pass of Re the presence of feed-back voltage on the plate of T1 may be eliminated by proper choice of the resistances R1 and R2, at the same time maintaining feed-back to T2.

The manner in which this takes place may be understood by reference to the equivalent bridge circuit shown in Fig. 2. From an examination of this bridge circuit it will be seen that if R1/R2 be made equal to RL/Rc, where R1. is the load impedance of T2 andthe other symbols have the same significance as above, it may be seen that the bridge is balanced so. that nofeed-back voltage appears across R4, that is no feed-back voltage appears between plate and ground of T1 and hence there is no distortion caused in the plate circuit of T1. The feed-back is still present insofar as T2 is concerned because there is feed-back voltage across R2 and therefore between grid and cathode of T2. Hence the addition of R2 of the proper magnitude results in elimination of distortion in T1.

Now in this condition there is nobypass across Re so that there will be voltage feed-back from that source also. are high compared with R2 which condition usually obtains, the feed-back factor due to R1 is R1 1 3.) R1 +R2 But l/ 2 L/ c so that R.+R2 RL+R. The feed-back factor due to Re is likewise i L+ c so that when the balanced bridge condition obtains there are two equal sources of feed-back to the grid of T2, and since both are in the same direction feed-back is doubled. Despite the fact that the feed-back with the circuit shown is higher than that due to the use of R1 alone, the distortion in the plate circuit of T1, which occurs when R1 only is used, is eliminated.

Another factor of importance is the effect of feed-back on the equivalent internal impedance If we assume that Rg and R2 of T2 looking back from the load. If the impedance of T2 without feed-back is R 2 then the equivalent impedance Z with feed-back and with an R R is given by +IL)RL where [L is the amplification factor of T2. With circuit values commonly encountered in practice the net effect is a reduction of equivalent plate impedance resulting in a desirable damping effect on the speaker.

This circuit thus serves to give increased feedback, eliminates distortion in T1 and results in increased speaker damping. In the type of circuit where feedback is due to Re alone (R1 omitted) distortion in T1 is not brought about but the circuit does not provide as much feedback as the circuit under consideration and it results also in a higher equivalent plate impedance and thus less speaker damping. The circuit according to my invention thus has all the advantages of the commonly used simple feedback circuits without their disadvantages.

While I have shown and described a preferred embodiment of the invention, it will be understood that modifications and changes may be made without departing from the spirit and scope of the invention, as will be understood by those skilled in the art.

What I claim is:

1. An amplifier circuit comprising an electron discharge tube having at least a cathode, a control grid and an anode, a first feed-back path connected between said anode and control grid, an un-bypassed cathode resistor providing a second feed-back path and a circuit including a resistance and a capacity serially connected between the cathode and grid of said tube.

2. An amplifier circuit comprising an electron discharge tube having at least a cathode, a control grid and an anode, a feed-back circuit including a resistance connected between said anode and control grid, an un-bypassed cathode resistor providing a second source of feed-back energy, and a circuit including a resistance and a capacity serially connected between the oathode and grid of said tube.

3. An amplifier circuit comprising a pair of coupled vacuum tube stages, means for feeding back energy from the output of the second stage to its input and incidentally unwanted feedback energy to the output of the first stage, and means for neutralizing said unwanted feed-back energy comprising an un-bypassed cathode resistor for the second stage and a series connection of a resistance and a capacity between the cathode and control grid of the second stage.

4. An amplifier circuit comprising a pair of coupled vacuum tube stages, means for feeding back energy from the output of the second stage to its input and incidentally unwanted feedback energy to the output of the first stage, means for neutralizing said unwanted feed-back energy comprising a series connection of a resistance and a capacity between the cathode and control grid of the second stage, and additional means for feeding back energy from the output of the second stage to its input, said last mentioned means comprising an un-bypassed cathode resistor for the second stage.

5. An amplifier circuit comprising a pair of resistance-capacity coupled vacuum tube stages, a feed-back circuit including a resistance for feeding back energy from the output of the second stage to its input and incidentally unwanted feed-back energy to the output of the first stage, and means for neutralizing said unwanted feedback energy comprising an un-bypassed cathode resistor for the second stage and a series connection of a resistance and a capacity between the cathode and control grid of the second stage, the feed-back resistance and the load resistance of the second stage constituting one pair of adjacent arms of a balanced bridge, and the unbypas'sed cathode resistor and the resistor of the series connection constituting the other pair of adjacent arms of said bridge.

6. -An amplifier circuit comprising a pair of resistance-capacity coupled vacuum tube stages, an un-bypassed resistor connected to the oathode of the second stage, a resistance connected from the anode of the second stage to the anode of the first stage and to the grid of the second stage for feeding back energy from the output of the second stage to its input and incidentally unwanted feed-back energy to the output of the first stage, and means for neutralizing said unwanted feed-back energy comprising a bridge network including as one pair of adjacent arms said feed-back resistance and the load impedance of the second stage, the common terminal of which is connected to the anode of the second stage, the other pair of adjacent arms including the un-bypassed cathode resistor of the second stage and a resistance-capacity combination connected between control grid and cathode of the second stage, the common terminal of the latter adjacent arms being connected to the cathode of the second stage, a resistance connected between the remaining terminals of said arms, one end thereof being connected to ground and the other end to the grid of said second stage, said bridge when in balanced condition permitting feed-back to take place between the output and input of the second stage but not between the outputs of the two stages.

DUDLEY E. FOSTER. 

